SUMMARY Endocrine-disrupting chemicals [e.g. Bisphenol A (BPA), BPS, phthalates, oxybenzone] are used in many household and personal care products (e.g., BPA is used in the coating of the inside surface of metal cans). These compounds can have estrogenic and inflammatory properties that could increase breast density. Breast density is one of the strongest risk factors for breast cancer. In a recent study, high circulating levels of BPA and monoethyl phthalate (MEP) were significantly associated with higher levels of mammographic breast density. For both compounds, breast density was about five percentage points higher among women with serum levels above the median detectable level compared with women with undetectable levels. A comparable percentage-point difference in breast density has been linked with a 5 to 10% increase in breast cancer risk in other epidemiologic studies. Concerns about the serum assay used in the BPA/phthalate-breast density study and the single time-point exposure measurement have led to controversy over its findings although it is not clear how a spurious positive association could have emerged. The fact that high MEP urinary levels were also associated with a four-fold increased risk of breast cancer in premenopausal women in a case-control study conducted in Mexico, and that a recent case-control study conducted in Canada observed striking associations between breast cancer risk in premenopausal women and employment in plastics manufacturing and in food canning further underscore the need for research in younger women. The proposed study focuses on the effects of environmental exposures on the breasts of young women during a window of increased susceptibility - before first childbirth. Breast density in young women has been shown to be as strongly associated with subsequent breast cancer risk as mammographic breast density in older women. Recent advances in computerized magnetic resonance imaging (MRI) to measure breast density have opened new avenues of research in young women because MRI does not carry the radiation risks associated with mammography and this approach eliminates inter-reader variation. Thus, we will examine the relation between urinary levels of endocrine-disrupting chemicals and breast density in 100 undergraduate female students enrolled at the University of Massachusetts Amherst. Non-enhanced magnetic resonance imaging for assessment of percent density volume will be conducted at the University of Massachusetts imaging facility. Urinary levels of BPA, BPS, BPF, 7 phthalate metabolites, oxybenzone, 4 parabens, triclosan, triclocarbon and 2 other phenols will be measured in a pooled urine specimen from 24-hour collections on three spaced days prior to imaging. The unique strengths of this study are the multiple time-point exposure measurements, the use of the urinary matrix to measure these chemicals, and computerized magnetic resonance imaging to measure breast density in young women.